In the absence of blind luck in stumbling upon a successful vaccine for HIV/AIDS, scientists will probably need to resolve a number of specific scientific obstacles in order to create a sufficient knowledge-base for informed vaccine design. The proposed studies are directed at delineating the targets of the strain-specific neutralizing antibody responses that follow HIV-1 infection and at achieving a better understanding of the strategies employed by HIV to resist neutralization by antibodies so that this information can be harnessed for improved immunogen design. Monkeys infected with SHIV-KB9 make antibodies that neutralize SHIV- KB9 but not SHIV-DH12. Conversely, monkeys infected with SHIV-DH12 make antibodies that neutralize SHIV-DH12 but not SHIV-KB9. This situation exactly paralells natural human infection with HIV-1 where the neutralizing response 3-10 months after infection is highly strain-specific. While it is reasonable to think that the variable loops of gp120 are principally responsible for this strain specificity, this has not been formally demonstrated. Furthermore, which variable loop or loops are the predominant determinant and whether they serve principally as the direct target of antibody recognition have not been elucidated. We will generate reagents and perform analyses that will allow precise delineation of the predominant determinants of the strain-specific neutralizing antibody response. These will include: variable loop swaps using SHIV-KB9 and SHIV-DH12; variable loop swaps using envelope genes from primary HIV-1 human infection; analysis of ability of these recombinant viruses to be neutralized by sequential serum samples from SHIV-KB9- and SHIV-DH12-infected monkeys and from the same HIV-1-infected people that will be the source of the envelope genes; analysis of the recognition sequences of neutralizing monoclonal Fabs from phage-display derived from the same sources. In addition to these studies, the extent to which certain N-linked carbohydrates shield CD4-binding.domain regions and CCRS-binding domain regions on gp120 to limit the neutralizing antibody response to more conserved determinants will be analyzed. Finally, a potential role for O-linked carbohydrates in shielding antibody access will be investigated. Results from these studies have the potential to enlighten immunogen design for the elicitation of neutralizing antibodies.